Conventional synthetic fertilizer materials such as urea, phosphatic fertilizer, potassium manure, ammonium sulfate and so forth, which are the most commonly used fertilizers, are readily soluble in water. Therefore, they are subject to leaching and their use results in a rapid release of their components. This loss of the fertilizer components will affect the sustained growth of crops and thus results in a decrease in harvest of the crops.
Many proposals have been made for obtaining a fertilizer product with sustained action, the releasing rate of the active components of which product can adequately be controlled.
In general, fertilizers with sustained action have been manufactured by chemical or physical methods. In chemical methods, a fertilizer material is reacted with one or more different compounds to produce another chemical substance, the active portions of which can steadly be degraded and released when the substance is applied to the ground.
The fertilizer products of this type involve urea-formaldehyde condensates disclosed in U.S. Pat. Nos. 2,830,036 and 3,227,543; crotylidenediurea and urea-acetaldehyde condensates in U.S. Pat. No. 3,190,741 and U.K. Pat. No. 1,041,537; isobutylidenediurea (IBDU) in U.S. Pat. No. 3,054,699 and Japanese Patent Publication No. (Sho) 38-7942; and magnesium ammonium phosphate in U.S. Pat. Nos. 2,827,368 and 3,181,943. However, these products require enormous production costs of about 3 to 4 times as compared with those of conventional products, because an additional procedure for reacting a fertilizer material with one or more different chemicals must be accompanied. Therefore, these chemical methods have not been commercially available.
Physical methods are divided into a combination method in which a conventional fertilizer material is entrained in and adsorbed on a proper material, and a coating method in which a thin-layer coating is formed around a granular fertilizer material.
In the combination method, wax, asphalt and petroleum resins (U.S. Pat. No. 3,219,433), high molecular weight compounds such as polyurethane (U.S. Pat. No. 3,232,739), white clay (U.S. Pat. No. 2,991,170) and the like are used as an entraining or adsorbing matrix for fertilizer materials. Although the fertilizer products in accordance with this combination method can be manufactured inexpensively, the products have drawbacks which the releasing rate of the fertilizer components is unequal and the content of the active ingredients per unit weight is lower.
Since, in accordance with the coating method, it is possible to manufacture a long-acting fertilizer product, which can contain higher fertilizer components, by selecting a proper coating material and to easily control the releasing rate of the components, the method has been considered most appropriate for practical use. Wax, high molecular weight compounds and sulfur have been proposed as coating materials useful in the coating method. Among the materials, was (U.S. Pat. Nos. 3,096,171 and 3,256,786) possesses physiochemically hydrophobic characteristics but there are some problems that the coating requires a plenty of wax which is apt to be adhered to the inner surfaces of the coating machines and the fertilizer material involved is readily diffused into wax.
A urea-aldehyde condensate (U.S. Pat. Nos. 3,214,259 and 3,248,255) among the high molecular weight compounds has been considered as an attractive coating material because it contains a high content of nitrogen. However, it has defects that the coating resulted from using the condensate as a coating material has a bad quality and high water-permeability. As another coating material, a copolymer between dicyclopentadiene and glycerol ester, which is available under the trademark OSMOCOTE from Archer Daniels Midland Company, U.S.A. (U.S. Pat. No. 3,233,518) is recommendable. By using a solution of the copolymer in a solvent as a coating material, it may be possible to form good multi-layer coatings around fertilizer particles. In practice, however, this material is expensively available and the solvent recovery remains unsolved.
A method for encapsulating a fertilizer material with sulfur is disclosed in U.S. Pat. No. 3,295,950 assigned to T.V.A., U.S.A. In this method, molten sulfur is sprayed on the surfaces of the fertilizer particles in a rotating drum. This method can provide coated fertilizers at lower costs because sulfur is cheaply available. However, the quality of the sulfur coating is poor and the method requires further various additives such as sealing agents for preventing the sulfur coating from being cracked and regulating agents for preventing the product from being coagulated and floating on the water surface. In addition, this sulfur-coated fertilizer product is very poor in views of its property and its storage quality. When the product is applied to the surface of ground, sulfur may easily be degradable by the action of the microorganisms living upon the soil and thus it is difficult to control the releasing rate of the fertilizer components into the soil.
From the foregoing, it will be noted that conventional long-acting fertilizer products have been encountered with many problems such as high production costs, imperfect coating ability and quality, recovery of the solvent used, prevention of the the biodegradation, decrease of the floatability on the water surface, storage quality, use of an excess amount of additivies, accumulation of the coating materials on and in the soil, the result, etc.